Intensive medical management of critically ill patients or patients undergoing operations which result in severe physiological disturbances requires detailed monitoring of the cardio-respiratory system. At present the apparatus used to provide such monitoring frequently require insertion into the body. For example, a pulmonary artery catheter must be inserted through the skin, into a major vessel and passed along the vessel, through the heart chambers into the pulmonary artery. The use of such monitoring devices add their own morbidity and mortality to the patient's underlying illness. These risks must always be weighed against the possible benefit of the information gained.
A non-invasive monitor would be of great benefit in that it would decrease the discomfort and risk of added morbidity and mortality to patients already very ill. It would further have application where such test information is required but invasive techniques are unjustified (in ambulatory patients and athletes) or impossible to institute (small infants).
In the prior art there are several non-invasive methods of obtaining PvCO.sub.2. These methods are based on the principle of using the lungs as a chamber that comes into equilibrium with the partial pressure of a gas dissolved in a liquid, that is, an aerotonometer, whereby the lung gases are brought into equilibrium with the gases dissolved in the inflowing pulmonary artery blood. This equilibrium must be reached before recirculated blood with altered gas concentrations reaches the lungs.